support ssl connections

[joprice]

I found that I couldn't use omigrate with cockroachdb, as it enforces ssl connections. It looks like pgx added ssl support, but only for the async driver. Since I was already using caqti in my project and was able to connect, I tried converting the postgres driver to it and have a working prototype: main...joprice:omigrate:caqti. There's a few pieces missing like not splitting statements on down commands, but I was able to successfully run it against a cockroachdb instance. Would there be any downsides to switching to this driver to allow for ssl connections? Or adding this implementation as a third option, perhaps by passing an arbitrary scheme (e.g. postgresql-ssl) that gets mapped back to postgresql when connecting?